As discussed hereinbelow, the invention is particularly concerned with, but is no way limited to, use in connection with tethered satellites being developed by the National Aeronautics and Space Administration (NASA). Such a tethered satellite, when released from the space shuttle carrying the satellite, serves as a source of potential energy. In particular, the gravity gradient between the satellite and the shuttle to which the satellite is tethered creates a tension in the tether. The latter is attached to a motor-driven reel and the mechanical energy produced, which is a function of tension times speed, must be converted into electrical energy by the motor-generator which drives and is driven by the reel. Moreover, on station, the motor must continually modulate the rate of movement of the satellite to achieve stability. This requires an inherent transition in the operation of the motor-generator between motor and generator operation and vice versa. The motor-generator functions as a motor to retrieve the satellite.
As will become apparent, the system of the invention is equally applicable to the bi-directional control of large inertia loads such as reaction wheels, energy storage flywheels, and electromechanical actuators for vane and elevon control. More generally, the system of the invention is applicable to situations wherein the motor must provide sudden stopping and reversal of load, e.g., in connection with cranes, industrial robots and the like.
Prior to the invention, two-phase brushless d.c. motors have been used when four quadrant control is required. There are a number of advantages of three-phase motor over a two-phase motor. For example, a two-phase motor requires one-third more power handling components than a three-phase motor. Further, the bandwidth of the current loop in a two-phase motor is determined by the number of poles and the speed, and this bandwidth may be greater than 1000 Hz in a high speed motor. With a three-phase motor (as with a conventional brush type motor), the bandwidth is determined by the system driven by the motor and this bandwidth is often less than 20 Hz.